Scala on Android

Android Development

● Write lots of Java code● Write UI and other resources in XML● Generate Java code from resources● Compile Java code into “.class” files● Run proguard● Compile class files to Dalvik Executable (DEX)● Deploy on device

Dalvik VM

● Does not support “.class” bytecode format● Can not use libraries which generate bytecode● Devices have limited CPU and Memory● Can not easily share libraries between

applications

Eclipse

● Has nice UI editors● IDE is a build tool

Alternative Languages for Android

Jython

● Does not have enough community support● Slow● Dynamic

JRuby

● Ruboto framework● Dynamic● Need to use java code generation

Clojure

● Very cool :)● Very slow start-up time● Sometimes need to generate bytecode

PHP

Who cares?

Rhino

● Nice language● Can run with optimization turned off● Completely interpreted● Dynamic

Groovy

???

Scala

● Statically typed● Compiled to Java bytecode● Has access to all classes in Android● Fast

● but sometimes slow :)

● Has SBT● Android● Maven artifacts

How to build Scala apps for Android

● ANT● Maven● SBT + Android Plugin

Little nice things about Scala

Type Inference

● Improve readability● Reduce number of imports● Less typing

val stream = getAssets().open("areas.json")

val stream: InputStream = getAssets().open("areas.json")

Import statement

import android.view.{

Window, Menu, MenuItem, KeyEvent, View

}

import android.util.Log.{e => error}

Lazy vals

lazy val searchBox =

findViewById(R.id.searchBox)

.asInstanceOf[AutoCompleteTextView]

Mixin Inheritance

class MainActivity extends MapActivity

with Logging

with DatabaseHelpers

with MapHelpers

Collections

● Best collection library I ever used● Sorry google-collections...

● Using arrays is easy● @specialized

Collection examples

val suggestions = areas map { _.name }

val index = areas.indexWhere(

_.name == searchString

)

val index =

areas.indexWhere(_.contains(coordinate))

pointList.minBy(_.lat)

First class functions

class AreaOverlay(

areas: Array[Area],

selectListener: Int => Unit

)

new AreaOverlay(

areas, this.onAreaSelected

)

Convert listeners into functions

implicit def fn2ClickListener(

f: View => Unit

): View.OnClickListener =

new View.OnClickListener() {

override def onClick(v: View) = f(view)

}

exitButton.setOnClickListener(

this.onExitClick

)

Files

● Directory structure is not forced to mirror packages

● Files can contain any amount of classes

Pattern matching

● No NPEs when using Option[T]● Easy to work with data objects

Pattern matching

lastLocation match {

case Some(c@Coordinate(lat, lon)) =>

...

case None =>

...

}

Implicits

implicit def coordinate2geopoint(

c: Coordinate

): GeoPoint = new GeoPoint(

c.lat * 1.0e6 toInt, c.lon * 1.0e6 toInt

)

XML

for (s <- xml \ "Document" \ "Style")

placemarkElem \ "Polygon" \

"outerBoundaryIs" \ "LinearRing" \

"coordinates"

Bad stuff

● Performance sensitive parts● Learn to use “javap”● Remove for-comprehensions and lambda functions● Do not use boxed types “by accident”

● Tools● UI Editor● Debugger● Build time (dex'ing of scala library)

What I missed from Java

● Google Guice● Maven● Good Eclipse integration● Feeling important...

Thank you!

Questions?